1. Technical Field
According to a first aspect thereof, the present invention relates to a rigid polyurethane foam preferably, while not exclusively, used for manufacturing composite building elements used in buildings or for manufacturing industrial constructions.
According to a second aspect thereof, the present invention relates to heat insulating building elements which incorporate or are coated with said foam, such us for instance composite panels for manufacturing ducts for heat ventilation and conditioning plants, curtain walls, elements for manufacturing civil or industrial roofing of the type used for buildings, heat insulating elements, such as for instance walls for cold-storage benches or rooms, or the same walls for civil or industrial constructions, suitably coated with said foam.
According to a further aspect thereof, the present invention also relates to a process for producing the aforementioned building elements.
2. Prior Art
As is known, rigid polyurethane foams are expanded polymers produced by a polyaddition mechanism between one or more compounds comprising two or more hydroxyl groups (such as, for instance, polyols) with one or more compounds comprising two or more isocyanate groups in the presence of a suitable foaming agent, whose presence is required to obtain the desired good heat insulating properties thanks to the formation of a foamed structure consisting of substantially closed cells.
Together with these properties, the rigid polyurethane foam should possess at the same time suitable adhesion characteristics to the supporting substrate(s) which constitute the building element to be manufactured, good mechanical characteristics, such as for instance low friability, low heat conductivity and, if so desired for the specific application, also good characteristics of dimensional stability on temperature changes, and/or of fire resistance.
The combination of these characteristics has been obtained for many years by using as foaming agent a class of halogenated hydrocarbons, namely chlorofluorocarbons (CFC), which however have been recently indicated by different parts as being the main cause of the progressive erosion of the ozone layer that protects earth atmosphere by the harmful UV-rays.
Because of the outlawing of CFCs due to their ozone depletion potential, the use of alternative foaming agents was proposed, among which some hydrocarbons have recently widespread in the practice, such as for instance n-pentane or cyclopentane, and hydrochlorofluorocarbons (HCFC) having an ozone depletion potential lower than CFCs"".
However, the use of such alternative foaming agents involves application limits which have not been overcome to date
Hydrocarbon foaming agents, in fact, while being, on the one hand easily adaptable to the existing foaming plants, on the other hand, are highly flammable and, in certain concentrations, apt to form explosive mixtures, which cause their use to be possible only with the adopt ion of strict safety measures, and only in some applications. So, for instance, the use of hydrocarbon foaming agents may be scarcely adopted in the practice for manufacturing composite panels wherein foam is interposed, as in a sandwich, between two metal sheets, since the sparks caused by cutting and shaping operations of the same might trigger dangerous combustion phenomena of the hydrocarbon trapped in the cellular structure of the foam.
Besides, the foams obtained by using hydrocarbon foaming agents show poor heat insulating characteristics in all those applications wherein heat insulation is an important parameter.
The use of hydrochlorofluorocarbon-based (HCFC) foaming agents, suitable per se to impart to the polyurethane foam the desired mechanical, adhesion, dimensional stability and heat insulation characteristics was, on the contrary, limited by the reduced commercial availability of these compounds, by their high cost and, not the least, by the approaching of the term beyond which also these halogenated hydrocarbons will not be utilizable any longer for environmental reasons.
In order to obviate this situation, it was attempted to use water as foaming agent, thanks to the presence in its molecule of hydrogen atoms having an acid character and, as such, capable of reacting with the isocyanate compound.
More particularly, water reacts with the isocyanate compound according to a multi-step reaction mechanism during which an unstable carbamic acid is formed which in turn decomposes, forming in the reaction medium gaseous carbon anhydride (which acts as the real foaming agent formed in situ) and an amine which, in turn, reacts with other isocyanate groups, leading to the formation of a compound including a disubstituted urea group.
However, the attempts made till now to use water as foaming agent, use which is certainly desirable due to reduced cost and practically unlimited availability of water, were disappointing.
In fact, it was experimentally found that by using water as foaming agent, the resulting foam is friable, dimensionally unstable on temperature changes, and, above all, has insufficient of adhesion characteristics to its supporting substrate.
The technical problem underlying the present invention is that of providing an improved rigid polyurethane foam which allows to overcome the limitations reported above with reference to the cited prior art.
According to a first aspect of the invention, the aforementioned problem is solved by a rigid polyurethane foam comprising a foamed structure consisting of closed cells substantially free therein of optionally halogenated hydrocarbon foaming agents, and having an adhesion strength to a supporting substrate, measured according to standards EN 1607, equal to or higher than 0.7 kg/cm2.
In the following description and in the appended claims, the terms: xe2x80x9cfoam consisting of closed cells substantially free therein of hydrocarbon foaming agentsxe2x80x9d is used to indicate a foam wherein no appreciable amounts of hydrocarbon foaming agents are found as such, nor possible residues originating by their use to carry out the expansion of the same foam.